WO2020007425A1

WO2020007425A1 - Electrolyte comprising a phospite as an additive or co-solvent, lithium rechargeable battery comprising said electrolyte, and method for producing the phosphite

Info

Publication number: WO2020007425A1
Application number: PCT/DE2019/200071
Authority: WO
Inventors: Volodymyr KOZEL; Natascha VON ASPERN; Gerd-Volker RÖSCHENTHALER; Martin Winter; Olesya STUBBMANN-KAZAKOVA; Isidora CEKIC-LASKOVIC; Christian WÖLKE
Original assignee: Forschungszentrum Jülich GmbH; Jacobs University Bremen Ggmbh
Priority date: 2018-07-06
Filing date: 2019-07-05
Publication date: 2020-01-09
Also published as: JP2021528806A; EP3818579B1; CN112470311A; JP7453158B2; US11901512B2; EP3818579A1; HUE059929T2; DE102018116475A1; US20210273263A1; PL3818579T3; DE102018116475B4

Abstract

The invention relates to an electrolyte, comprising: at least one lithium salt; at least one first and one second organic solvent, the first organic solvent having a dielectric constant of more than 90 (at 40°C), and the second organic solvent having a boiling point of below 110°C; and at least one phosphite of formula (I), where R₁ is a n-propoxy group, an iso-propoxy group, a tert-butoxy group, an n-pentafluoropropoxy group, an n-trifluoropropoxy group, an iso-hexafluoropropoxy group, or a tert-nonafluorobutoxy group, and R₂, R₃, R₄ and R independently of one another, are selected from among H, a trifluoromethyl group, or a C₂-C₆-alkyl group which is substituted by a trifluoromethyl group. The invention also relates to the use of an electrolyte comprising a phosphite of formula (I), and to a lithium rechargeable battery comprising said electrolyte.

Description

Electrolyte with a phosphite as an additive or co-solvent, lithium secondary battery with this electrolyte and process for producing the phosphite

Field of the Invention

The invention relates to an electrolyte with a phosphite as an additive or co-solvent, a lithium secondary battery with this electrolyte and a method for producing the phosphite.

Background of the Invention

[02] In order to realize the efficient use of renewable energies and for use in electromobility, the importance of batteries as an energy source and storage medium has increased significantly in recent years. However, in order to be able to achieve the desired goals, the batteries must be continuously improved.

[03] In all types of current and future lithium, lithium-ion and dual-ion batteries, the electrolytes, because of their interaction with all other active and inactive materials used, have a key function in terms of performance, life and safety. The liquid electrolyte has a significant impact on a variety of chemical and technological aspects of a battery. The spectrum ranges from the conductivity - due to the dissolving power of the conductive salts - to the electrochemical stability, the usability, the flammability and the vapor pressure up to the film-forming properties of the electrolyte, the main effects on the so-called "Solid Electrolyte Interface" (SEI) formation at the anode or the “cathode Electrolyte Interface "(CEI) in training at the cathode.

[04] The liquid electrolyte as the core component of each battery cell consists of a conductive salt, such as LiPF _6, and a mixture of selected linear and cyclic carbonates (generally referred to as the standard electrolyte).

[05] A major weak point of such electrolytes is the low oxidative stability and the high flammability (Xu, K. Chem. Rev. 2004, 104 (10), 4,303 - 4,418).

[06] Oxidative stability is a major problem, in particular, when higher final charge voltages are used due to the development of new cathode materials (5 V cathodes) and thus the energy density is increased (Xu, K., Chem. Rev. 2014, 114 (23), 11503-11618).

[07] When using a final charge voltage above 4.3 V against a lithium reference (Li / Li ⁺ ), there is an oxidative decomposition of the carbonate-based electrolyte at the cathode, which results in an increased decrease in capacity and thus the life of the battery reduced.

[08] One approach to prevent the decomposition of the electrolyte at the cathode and the consequent decrease in capacity is the use of additives in electrolytes (usually in an amount of 0.1 to 5% by weight). The advantage of this approach is that the main structure of the electrolyte is preserved. The role of the additive is to form a protective layer (CEI) on the cathode and thus the electrolyte _Z replacing the standard electrolyte to prevent or reduce (Xu, K., Chem. Rev. 2014, 114 (23), 11503- 11618 ). [09] A whole series of different additives are known for this from the prior art. On the one hand, these are partially fluorinated organic compounds that contain no heteroatoms apart from fluorine (Zheng, X .; Huang, T .; Pan, Y .; Wang, W .; Fang, G .; Ding, K .; Wu, M. , ACS Applied Materials & Interfaces 2017, 9 (22), 18758-18765; Kim, S .; Kim, M.; Choi, I .; Kim, JJ, J. Power Sources 2016, 336 (Supplement C), 316- 324), as well as partially fluorinated organic compounds that additionally contain the following heteroatoms: nitrogen (see, for example, Wang, L .; Ma, Y .; Li, Q .; Zhou, Z .; Cheng, X .;

Zuo, P .; Du, C .; Gao, Y .; Yin, G., J. Power Sources 2017, 361 (Supplement C), 227-236), phosphorus (see e.g. from Aspern, N .;

Röser, S .; Rezaei Rad, B .; Murmann, P .; Streipert, B .;

Mönnighoff, X .; Tillmann, S. D .; Shevchuk, M.; Stubbmann-

Kazakova, 0 .; Röschenthaler, G.-V .; Nowak, S .; Winter, M.;

Cekic-Laskovic, I., J. Fluor. Chem. 2017, 198 (Supplement C),

24-33; Tornheim, A.; He, M.; Su, C.-C .; Zhang, Z., J.

Electrochem. Soc. 2017, 164 (1), A6366-A6372; Wagner, R .;

Korth, M.; Streipert, B .; Kasnatscheew, J .; Gallus, D. R .;

Brox, S .; Amereller, M.; Cekic-Laskovic, I .; Winter, M., ACS Applied Materials & Interfaces 2016, 8 (45), 30871-30878),

Sulfur (see e.g. Rong, H .; Xu, M.; Zhu, Y .; Xie, B .; Lin, H .; Liao, Y .; Xing, L .; Li, W., J. Power Sources 2016, 332

(Supplement C), 312-321); Boron (see e.g. Wang, K .; Xing, L .; Zhu, Y .; Zheng, X .; Cai, D .; Li, W., J. Power Sources 2017, 342 (Supplement C), 677-684) ; Silicon (see e.g. Wang, K .;

Xing, L .; Zhu, Y .; Zheng, X .; Cai, D .; Li, W., J. Power Sources 2017, 342 (Supplement C), 677-684; Imholt, L .; Roeser,

S .; Börner, M.; Streipert, B .; Rad, B. R .; Winter, M.; Cekic-Laskovic, I., Electrochim. Acta 2017, 235 (Supplement C), 332-

339).

[10] Organometallic (Imholt,

L .; Röser, S .; Börner, M.; Streipert, B .; Rad, B. R .; Winter,

M.; Cekic-Laskovic, I., Electrochim. Acta 2017, 235 (Supplement C), 332-339) and lithium salt-based (see e.g. Dong, Y .; Young, BT; Zhang, Y .; Yoon, T .; Heskett, DR; Hu, Y .; Lucht, BL, ACS Applied Materials & Interfaces 2017, 9 (24), 20467-20475) Electrolyte additives proposed.

[11] By adding these additives to the standard electrolyte, the decrease in capacity can be reduced but not completely prevented. All of these additives decompose on oxidation at the cathode in front of the standard electrolyte and form an effective CEI. By this CEI, the more electrolyte _Z substitution prevents the internal resistance is reduced, improving the structural stability of the cathode, inhibits the formation of gases and the metal dissolution of the metals contained in the cathode and the ion mixture of Li ⁺ - and Ni ²⁺ ions in the cathode can be minimized due to similarly large ion radii.

[12] In addition to the oxidative decomposition at high final charge voltages, the safety aspect, in particular the flammability, of an carbonate-based electrolyte is an important problem. The linear and cyclic carbonates which are usually used are highly flammable; there are always reports of the occurrence of battery fires, such as on the Samsung Galaxy Note 7 in 2016. For this reason, it is important to develop safer batteries with regard to flammability.

[13] In addition to the development of new solvents for electrolytes, it is possible to add flame retardants as additives (<5% by weight) or co-solvents (> 5% by weight) to the standard electrolyte (Xu, K., Chem. Rev . 2004, 104 (10), 4303-4418; Xu, K., Chem. Rev. 2014, 114 (23), 11503-111618). With this approach, too, the main structure of the electrolyte is not changed.

[14] Compounds which have phosphorus atoms in oxidation state III or V have proven to be effective flame retardants contain. Compounds with phosphorus atoms in oxidation state V can be divided into two subgroups: on the one hand, phosphates / phosphites, on the other hand phosphazenes, which in addition to several phosphorus atoms also contain several nitrogen atoms.

[15] It has been shown that the use of phosphites (oxidation level III) and phosphates (oxidation level V) as flame retardants requires a high proportion (> 20% by weight) in order to obtain a non-combustible electrolyte (see e.g. from Aspern, N .; Röser, S .; Rezaei Rad, B .; Murmann, P .; Streipert, B .; Mönnighoff, X .; Tillmann, SD; Shevchuk, M.; Stubbmann-Kazakova, 0 .; Röschenthaler , G.-V .; Nowak, S .; Winter, M.; Cekic-Laskovic, I., J. Fluor. Chem. 2017, 198 (Supplement C), 24-33; Murmann, P .; Mönnighoff, X .; von Aspern, N .; Janssen, P .; Kalinovich, N .; Shevchuk, M.; Kazakova, 0 .; Röschenthaler, G.-H .; Cekic-Laskovic, I .; Winter, M., J. Electrochem. Soc. 2016, 163 (5), A751-A757). However, the high proportion of flame retardants affects the cyclization (performance and service life) of the battery in a negative sense compared to the standard electrolyte. This is primarily due to the incompatibility of the flame retardants used with the graphitic anode.

[16] In order to improve the incompatibility of these flame retardants with the graphitic anode, the replacement of the hydrogen atoms with fluorine atoms has proven to be an effective approach. This approach significantly improved the cyclization and thus the service life of the battery compared to the non-fluorinated co-solvent. However, the performance and lifespan of the standard electrolytes could not be achieved (Murmann, P .; Mönnighoff, X .; von Aspern, N .; Janssen, P .; Kalinovich, N .; Shevchuk, M.; Kazakova, 0 .; Röschenthaler, G.-H .; Cekic-Laskovic, I .; Winter, M., J. Electrochem. Soc. 2016, 163 (5), A751-A757; Mönnighoff, X .; Murmann, P .; Weber, W .; Winter, M.; Nowak, S., Electrochim. Acta 2017, 246 (Supplement C), 1042-1051).

[17] Another advantage of introducing fluorine atoms is that they also have an influence on the flammability, which means that fluorinated compounds can in principle reduce the flammability of the electrolyte better than non-fluorinated compounds. However, the flame chemistry of fluorinated compounds is complex and not yet fully understood, so the degree of fluorination does not allow any conclusions to be drawn about the flammability of individual compounds (Murmann, N. von Aspern, P. Janssen, N. Kalinovich, M. Shevchuk, G.- V. Röschenthaler, M. Winter, I. Cekic-Laskovic, J. Electrochem. Soc. 2018, 165, A1935-A1942; P. Murmann, X. Mönninghoff, N. von Aspern, P. Janssen, N. Kalinovich, M Shevchuk, 0. Kazakova, G.-V. Röschenthaler, I. Cekic-Laskovic, M. Winter, J. Electrochem. Soc. 2016, 163, A715-A757).

[18] By using phosphazenes as flame retardants, the proportion (5% by weight) can be drastically reduced in order to obtain a non-combustible liquid electrolyte, with no negative influence on the cyclization. Furthermore, these additives can also be used for high-voltage applications because they form an effective CEI (Ji, Y .; Zhang, P .; Lin, M.; Zhao, W .; Zhang, Z .; Zhao, Y .; Yang, Y., J. Power Sources 2017, 359 (Supplement C), 391-399; Xia, L .; Xia, Y .; Liu, Z., J. Power Sources 2015, 278 (Supplement C), 190-196) ,

[19] This bifunctional application is not known for phosphites and phosphates. However, it has already been shown that by varying the proportion of phosphites / phosphates in the standard electrolyte, different applications can be realized. With a small proportion (1% by weight), the phosphites / phosphates can be used as high-voltage additives, with a high proportion (> 20% by weight) they serve as Flame retardants (von Aspern, N .; Röser, S .; Rezaei Rad, B .; Murmann, P .; Streipert, B .; Mönnighoff, X .; Tillmann, SD; Shevchuk, M.; Stubbmann-Kazakova, 0 .; Röschenthaler, G.-V .; Nowak, S .; Winter, M.; Cekic-Laskovic, I., J. Fluor. Chem. 2017, 198 (Supplement C), 24-33). However, no electrochemical measurements for the high phosphite / phosphate content were carried out in this work, so that no statement can be made regarding the performance and service life of the batteries.

[20] In the investigations of the standard electrolyte after electrochemical aging, different phospholanes with oxidation level V were observed as decomposition products (Takeda, S .; Morimura, W .; Liu, Y.-H .; Sakai, T .; Saito, Y., Rapid Commun. Mass Spectrom. 2016, 30 (15), 1754-1762; Weber, W .; Kraft, V .; Grützke, M.; Wagner, R .; Winter, M.; Nowak, S., J. Chromatogr A 2015, 1394 (Supplement C), 128-136). In order to investigate the influence of these decomposition products on the standard electrolytes, phospholanes as solvents, co-solvents or additives were examined and showed a positive influence on the general electrochemical performance of the battery. When used as a solvent together with ethylene carbonate (EC), phospholanes with oxidation levels III and V were investigated, the end group of the alkyl group having 0 to 3 fluorine atoms. This system was postulated to be in Li / LMO cells with a final charge voltage of 4.2 V vs. Li / Li ⁺ behaves similarly to the cyclic phosphates and that it is a fire retardant solvent (Narang, SC; Ventura, SC; Zhao, M.; Smedley, S .; Koolpe, G .; Dougherty, B. W097 / 44842 ).

When using a co-solvent, such as ethylene ethyl phosphate (EEP), which contains no fluorine atoms, the flammability of the electrolyte can be reduced by approximately 50% by adding 10% by weight. Furthermore, by the formation of a protective film on the anode and cathode, an improvement in the cyclization at a final charge voltage of 4.3 V vs. Li / Li ⁺ can be achieved compared to the standard electrolyte (Gao, D .; Xu, JB; Lin, M .; Xu, Q .; Ma, CF; Xiang, HF, RSC Adv. 2015, 5 (23), 17566-17571 ).

[22] Other phospholanes with oxidation level III have also been investigated as co-solvents and additives. The length and degree of fluorination of the alkyl group were varied and the ring was substituted by one or two alkyl chains, the length and degree of fluorination also being varied here. An approximately 30% decrease in the flammability compared to the standard electrolyte was shown, with an addition of 30% by weight of the phospholane. When a half cell was galvanostatically cycled at 0.001 V to 1.5 V, the cells which contained a phospholane as cosolvent showed a good charging and discharging profile. However, the capacities achieved are still significantly below that of the standard electrolyte. When used as an additive, better capacities than the standard electrolyte could again be achieved (US 8062796 B2). A test on full cells was not carried out.

[23] However, it is known that measurement results from half cells cannot easily be transferred to full cells (V. Sharova, A. Moretti, T. Diemant, A. Varzi, RJ Behm, S. Passerini, J. Power Sources (2018 ), 375, 43-52). It is also known from the prior art that cells show a significantly different behavior when using the final discharge voltage up to 4.2 V than when using the final discharge voltage above 4.2 V (high-voltage applications) (J. Li, H. Liu, J. Xia, AR Cameron, M. Nie, GA Botton. JR Dahn, J. Electrochem. Soc. (2017), 164, A655-A665; J. Li, SL Glazier, K. Nelson, X. Ma, J. Harlow, J. Paulsen , JR Dahn, J. Electrochem. Soc. (2018), 165, A3195-A3204). [24] It could also be shown that when EEP (5% by weight) is added as an additive to an electrolyte reference system made of ethylene carbonate: diethyl carbonate: trimethyl phosphate (6: 2: 2), the problem of phosphate reduction on the anode is eliminated can be formed by the formation of a protective layer on the anode, induced by EEP, and thus the cyclization at a final charge voltage of 1.5 V vs. Li / Li ⁺ can be improved (Ota, H .; Kominato, A.; Chun, W.- J .; Yasukawa, E .; Kasuya, S., J. Power Sources 2003, 119

(Supplement C), 393-398).

[25] In addition, EEP (when adding 1% by weight) can also be used for

High-voltage applications are used. Here, the cell for ten charge / discharge cycles up to 4.4 V vs. Li / Li cyclized at 20 ° C, then the cell was stored for 2 h at 45 ° C, then to 4.6 V vs. Li / Li ⁺ to charge and then hold for 60 h. This showed that EEP forms a CEI and has a lower internal resistance compared to standard electrolytes (Tornheim, A.; He, M.; Su, C.-C .; Zhang, Z., J. Electrochem. Soc. 2017 , 164 (1), A6366-

A6372). However, these tests cannot show how frequent charging and discharging with high end-of-charge voltages affects the performance of the battery.

The invention is therefore based on the object of further improving previously known electrolytes for lithium secondary batteries, in particular reducing the flammability and / or improving the performance of the batteries even in high-voltage applications.

Summary of the invention

According to the invention, this object is achieved by an electrolyte comprising: at least one lithium salt; at least first and second organic solvents, the first organic solvent having a dielectric constant greater than 90 (at 40 ° C) and the second organic solvent having a boiling point below 110 ° C; and at least one phosphite of the formula (I)

wherein Ri is an n-propoxy group, an iso-propoxy group, a tert-butoxy group, a Ώ-pentafluoropropoxy group, an n-trifluoropropoxy group, an iso-hexafluoropropoxy group or a tert-nonafluorobutoxy group and R ₂ , R ₃ , R ₄ and R _{5 are} independent are selected from each other from H, erner

Trifluoromethyl group or a C ₂ -C ₆ alkyl group which is substituted with a trifluoromethyl group.

[28] An n-propoxy group means the group H ₃ C-CH ₂ -CH ₂ -0-, an iso-propoxy group means the group [CH ₃ ] ₂ -CH- 0-, a tert-butoxy group means the group [CH ₃ ] ₃ -C-0-, under an n-pentafluoropropoxy group the group F ₃ C-CF ₂ -CH ₂ -0-, under an n-trifluoropropoxy group the group F ₃ C-CH ₂ -CH ₂ -0- , under an iso-hexafluoropropoxy group the group [CF ₃ ] ₂ -CH-0- and under a tert-nonafluorobutoxy group the group [CF ₃ ] ₃ -C- 0-.

[29] The electrolytes according to the invention have a significantly reduced flammability.

To achieve a reduced flammability, a smaller amount of phosphite additive is necessary than that of the compounds known from the prior art. This can cause non-combustibility with an additive content of 15 % By weight are achieved, whereas an additive proportion of at least 30% by weight is necessary for a comparable effect in the prior art (US Pat. No. 8062796 B2).

[31] The reduced flammability can be achieved in particular by substituting the phosphite ring with trifluoromethylethylene groups, as has been achieved, for example, in the compound PFPOEPi-ICF ₃ .

The following abbreviations are used in the following:

P

[33] A substance is considered combustible if it has a burning time of> 20 s / g. With a burning time between 6 and 20 s / g one speaks of flame retardancy, <6 s / g of non-

Flammability.

[34] A "normal range" can be understood to mean a cut-off voltage of up to 4.2 V and a "high-voltage range" means voltages above 4.2 V, in particular a voltage of 4.5 V. [35] An “improved cyclization behavior”, “advantageous cyclization behavior” or “positive cyclization behavior” is understood to mean a comparatively higher preservation of a capacity of a cell with the same number of charge and discharge cycles.

[36] Furthermore, the electrolytes according to the invention show improved cyclability. This applies in particular to the use of additives in which R _{x is} an aliphatic unsubstituted group and to cyclizations above 4.2 V. Until now, the use of phosphites with the structures shown or dioxaphospholanes in the high-voltage range (above 4.2 V) was not yet known.

[37] It was also found that surprisingly advantageous cyclization results can be achieved using unfluorinated phosphites. To date, attempts have been made in the prior art to improve the cyclization by means of fluorination.

[38] Since the measurements are carried out in full cells, it can be shown that the compounds according to the invention can contribute to a positive cyclization behavior both on the cathode and on the anode.

[39] Preferably, Ri is an n-propoxy group, an iso-propoxy group or a tert-butoxy group.

[40] In a preferred embodiment, Ri is a tert-butoxy group.

[41] Preferably Ri is an n-pentafluoropropoxy group.

[42] In a preferred embodiment, R2 is a trifluoromethyl group and R ₃ , R ₄ and R ₅ H.

[43] In another embodiment, R ₂ , R ₃ , R ₄ and R _{5 are} H. [44] In a preferred embodiment, the at least one phosphite is present in a proportion of 0.1 to 15% by weight, based on the total weight of the organic solvents.

[45] The invention further relates to the use of an electrolyte in a lithium secondary battery at a voltage greater than 4.2 V, the electrolyte comprising: at least one lithium salt; at least a first and a second organic solvent, the first organic solvent having a dielectric constant of more than 90 (at 40 ° C) and the second organic solvent having a boiling point of less than 110 ° C; and at least one phosphite of the formula (I)

wherein Ri is a fully or partially halogen-substituted, linear or branched C2-C20-alkyl group and R ₂ , R ₃ , R ₄ and R ₅ are selected independently of one another from H or an unsubstituted or fully or partially halogen-substituted, linear or branched Ci-C2o ^_ Alkyl group.

The invention also relates to a lithium

Secondary battery comprising a cathode; an anode; a separator; and the electrolyte according to one of claims 1 to 7.

The proportion of phosphite in the electrolyte is preferably 0.1 to 15% by weight, based on the total weight of the organic solvents.

[47] The at least one phosphite is preferably present in a proportion of 1-30% by weight, based on the total weight of the organic solvents.

[48] The lithium salt is preferably selected from the group consisting of lithium perchlorate (LiCICy), lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), lithium hexafluoroarsenide (LiAsFe), lithium trifluorometasulfonate (LZCF ₃ SO ₃ ), lithium bis-tronifluoromethyl LiN (CF ₃ SO ₂ ) ₂ ), lithium bis (oxalato) borate (LiB [C ₂ O ₄ ] ₂ ), lithium oxalyl difluoroborate (LiBF ₂ C ₂ 0 ₄ ), lithium nitrate (LiNCg), lithium tri-perfluoroethyl perfluorophosphate (LiPF ₃ (CF ₂ CF ₃ ) ₃ ), lithium bis-perfluoroethylsulfonylimide (C ₄ FioLiN0 ₄ S ₂ ), lithium bis (trifluoromethanesulfonyl) imide (CF ₃ SO ₂ NLZSO ₂ CF ₃ ), lithium bis (fluorosulfonyl) imide ( F ₂ N0 ₄ S ₂ Li), lithium trifluoromethanesulfonimide (C ₂ F ₆ LiN0 ₄ S ₂ ) and mixtures thereof.

In a preferred embodiment of the invention, the first organic solvent is selected from the group consisting of ethylene carbonate, polypropylene carbonate, butylene carbonate, 2, 3-butylene carbonate, isobutylene carbonate, propoylene carbonate, g-butyrolactone and mixtures thereof.

[50] In a further preferred embodiment of the invention, the second organic solvent is selected from the group consisting of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, dimethoxyethane, diethoxyethane, aliphatic ester derivatives and mixtures thereof. [51] The first organic solvent and the second organic solvent are preferably present in a ratio of 5: 1 to 1: 5, more preferably 2: 1 to 1: 2, particularly preferably 1: 1.

[52] The invention further relates to a lithium secondary battery comprising a cathode; an anode; a separator; and the electrolyte according to the invention.

The term “secondary battery” in the context of this invention is understood to include any rechargeable electrical storage.

Preferably, the active material of the cathode consists of a material which is selected from lithium-manganese-nickel oxides, lithium-nickel-manganese-cobalt mixed oxides of the LiMCy type with M = Mn, Co, Ni or of the LiMPCy type with M = Ni, Mn, Co, and lithium-rich transition metal oxides of the type xLiMn0 ₃ - (l- x) LiM0 ₂ with M = Ni, Mn, Co, and 0 <x <1.

[54] In a preferred embodiment, the lithium secondary battery is made of a material selected from the group consisting of lithium metal, graphite, silicon and alkali or alkaline earth metals.

[55] In one embodiment of the invention, the proportion of phosphite in the organic electrolyte is 1-5% by weight, based on the total weight of the organic solvents. In this case, the phosphite according to the invention is used as an additive. [56] In another embodiment of the invention, the proportion of phosphite in the organic electrolyte is 10-30% by weight, based on the total weight of the organic solvents. In this case, the phosphite according to the invention therefore serves as a co-solvent.

The invention further relates to a process for the preparation of a phosphite of the formula (I), in which in a one-pot reaction in a first step a phosphorus (111) halide, preferably phosphorus (III) chloride, with a vicinal diol in the presence a base, preferably triethylamine, is reacted to give a 2-halo-1,3-dioxaphospholane, and then in a second step esterification with an alcohol in the presence of a base, preferably triethylamine, takes place to give a phosphite of the formula ( I) to yield.

[58] It was shown that the proposed phosphites, which were successfully produced for the first time in a one-pot synthesis, can be used in a variety of ways as additives or cosolvents for electrolytes in lithium secondary batteries. The correct selection of the concentration of phosphite in the electrolyte determines the application.

[59] When used as an additive (<5% by weight) with a final charge voltage> 4.4 V vs. Li / Li ^{+ leads} to the formation of an effective CEI on the cathode and the cyclization can be significantly improved. The longevity of the batteries can thus be improved by at least 100 cycles compared to the standard electrolyte, since the decrease in capacity is reduced.

The synthesis process according to the invention for the phosphites used in the electrolyte according to the invention is based on a phosphorus (III) halide (preferably PCI ₃₎ and a vicinal diol in the presence of a base such as pyridine or Triethylamine in THF as the preferred solvent. The resulting 2-halogenated 1,3-dioxaphsopholane is esterified in a second step of the one-pot synthesis with an alcohol in the presence of base, preferably triethylamine in THF.

[61] Furthermore, the battery can be improved by selecting a high concentration (10-15% by weight) of the phosphite. The flammability of the electrolyte can be completely suppressed. In addition, the addition of the phosphite at a final charge voltage of 4.2 V vs. Li / Li ⁺ the cyclization clearly compared to that

Standard electrolytes can be improved. This results in a longer-lasting and safer battery.

[62] In a further embodiment, the phosphite (I) is substituted with a trimethylsilyl group. The cyclization can be improved by adding a trimethylsilyl-substituted compound. Without wishing to be limited by theory, it is assumed that the trimethylsilyl group intercepts hydrogen fluoride formed as an intermediate and thereby avoids damage to the battery which otherwise occurs.

[63] The invention further relates to the use of PFPOEPi-lCF3 in an electrolyte at 0.1 V up to 4.2 V.

The invention also relates to the use of an electrolyte in a lithium secondary battery at a voltage of 4.2 V or less than 4.2 V, the electrolyte comprising:

at least one lithium salt; at least a first and a second organic

Solvents, wherein the first organic solvent has a dielectric constant of more than 90 (at 40 ° C) and the second organic solvent one

Boiling point below 110 ° C; and at least one phosphite of the formula (I)

where Ri is an n-pentafluoropropoxy group, R _{2 is} a

Is trifluoromethyl group and R ₃ , R ₄ and R _{5 are} H.

[65] By adding compound (I) to the electrolyte, an advantageous cyclization behavior can be made possible.

Detailed description of the invention

[66] Further advantages result from the attached drawing and the following examples. It shows

Fig. 1 is a diagram with the results of a SET (Self-Extinguishing Time) test of an electrolyte with IM LiPF ₆ in ethylene carbonate and ethyl methyl carbonate (1: 1). Here 500 mΐ of the electrolyte to be examined were applied to a glass fiber filter GF / C (Whatman Separator) (0 25 mm). This was placed in the measuring device and then the “self-extinguishing time” was determined with the aid of a UV-Vis detector.

Examples

Preparation example

[67] Preparation of 2- (2, 2, 3, 3, 3-pentafluoropropoxy) -4

(trifluoromethyl) -1, 3, 2-dioxaphospholane (Scheme 1)

Scheme 1

All reaction steps take place in an argon atmosphere in dry solvents.

15.0 g (0.12 mol, 1 eq.) 3, 3, 3-trifluoropropane-1,2-diol (XJ Wang et al. / Electrochemistry Communications 12 (2010) 386-389) is combined with NEt ₃ (23 , 3 g, 0.24 mol, 2 eq.) And at 0 ° C. to dissolve PCI3 (16.4 g, 0.12 mol, 1 eq.) In CH2Cl2 / Et20 (10: 1) (250 ml ) slowly added dropwise. The ice bath was removed and the reaction mixture brought to room temperature. The course of the reaction was monitored by ³¹ P-NMR. The reaction was complete after 48 h, the signal from 2-chloro-4- (trifluoromethyl) -1, 3, 2-dioxaphospholane being observed at 172.78 ppm (approx. 70%). The product was filtered with a reverse frit under argon, the residue was washed with ether and the filtrate was reacted further without purification. 18.0 g (0.12 mol, 1 eq.) 2, 2, 3, 3, 3-pentafluoropropanol with NEt ₃ (12.12 g, 0.12 mol, 1 eq.) Were added dropwise to the filtrate (room temperature, Ar). The course of the reaction was monitored by means of ³¹ p-NMR. The implementation was complete after 24 hours. Two diastereomers of 2- (2, 2, 3, 3, 3-pentafluoropropoxy) -4- (trifluoromethyl) -1, 3, 2-dioxaphospholane and approx. 30% by-products were observed. The reaction solution was filtered with a reverse frit under argon, the residue was washed with ether, the solvent was stripped off and the residue was distilled (52 mbar). The diastereomer mixture of 2- (2, 2, 3, 3, 3-pentafluoropropoxy) - 4- (trifluoromethyl) -1, 3, 2-dioxaphospholane (Scheme 2) is a colorless viscous liquid, b.p. 44 ° C (52 mbar ). Yield: 5.3 g, 0.016 mol = 14%

OCH ₂ CF ₂ CF ₃ OCH ₂ CF ₂ CF ₃

0 ^A 0

F, Cy F, Cy

Scheme 2

¹ H-NMR (400 MHz, CDC1 ₃ ): d 4.79 - 4.69 (m, 1H), 4.54 - 4.45 (m,

1H), 4.41 - 4.34 (m, 1H), 4.33 - 4.15 (m, 5H).

³¹ P NMR (162 MHz, CDC1 ₃ ): d 145.43 (t, J = 7.2 Hz), 144.06 (t, J = 6.7 Hz).

¹⁹ F-NMR (376 MHz, CDC1 ₃ ): d -76.22 (s), -79.31 (s),

83.33 (s), -83.43 (s), -83.50 (s), -124.28 (s), -124.42 (t,

J = 11.9 Hz), -126.03 (t, J = 14.6 Hz).

applications

To study the behavior of the electrolyte according to the invention for high-voltage applications, an electrolyte with IM LiPF ₆ in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (1: 1) was used in one case without the addition of phosphite, in the other case with the addition of the stated amount Phosphite (% by weight) in graphite / NCMlll cells at a given voltage vs. Li / Li ⁺ tested. A microporous ceramic-coated film (available under the brand name Separion from Evonik Industries AG) was used as the separator. The cells were formed with a C rate of 0. IC and a subsequent C rate of 0.5C for 3 cycles each at the voltage described below. The C rate describes the charge (or discharge) current of a battery, based on the capacity C. A constant voltage step of 3 hours is integrated in each of the formation cycles. The cells were then cycled at a C rate of IC. The results of measurements at 4.5 V are shown in Table 1.

[69] In the following tables, the "Electrolyte additive" header indicates the percentage of a phosphite in% by weight that was added as an additive to the electrolyte used, "80% SOH" the 80% capacity retention of the 130th cycle in percent on (the respective cycle achieved is in brackets), "Efficiency [%]" indicates the first cycle efficiency in percent and "Retention [%]" indicates the capacity retention in percent after 100 cycles.

Table 1

[70] Further measurements were carried out at 4.2 V, which are listed in Table 2.

Table 2

[71] In order to test the influence of phosphite on the flammability of the electrolyte, a SET (Self-Extinguishing Time) test was carried out on an electrolyte according to the invention with IM LiPF ₆ in EC: EMC (1: 1) (FIG. 1) , The volume of the tested electrolyte was 500 mΐ. UV-VIS was used as the detector.

[72] Table 3 shows two to five times (in

Seconds) which were determined using the SET test. The measurements relate to a 1M mixture of LiPF ₆ EC: EMC (1: 1) with the percentage (% by weight) of the compound indicated in the left line in the header of the table. The proportion of the specified phosphite ranged from 0% by weight to 15% by weight. For fields with one, none

Measurement carried out.

Table 3

[73] A significant decrease in the flammability can already be seen from a phosphite content of 10% by weight. With a phosphite content of 15% by weight, depending on the phosphite added, the electrolyte could be classified as flame-retardant to non-combustible.

[74] The features of the invention disclosed in the above description, in the claims and in the drawing can be essential both individually and in any combination for the implementation of the invention in its various embodiments.

Claims

Claims:

1. An electrolyte comprising: at least one lithium salt; at least a first and a second organic

Boiling point below 110 ° C; and at least one phosphite of the formula (I)

wherein Ri is an n-propoxy group, an iso-propoxy group, a tert-butoxy group, an n-pentafluoropropoxy group, an n-trifluoropropoxy group, an iso-hexafluoropropoxy group or a tert-nonafluorobutoxy group and R ₂ , R ₃ , R ₄ and R _{5 are} independent are selected from each other from H, one

Trifluoromethyl group or a C2-C6 alkyl group which is substituted with a trifluoromethyl group.

2. Electrolyte according to claim 1, characterized in that Ri is an n-propoxy group, an iso-propoxy group or a tert-butoxy group.

3. Electrolyte according to claim 2, characterized in that Ri is a tert-butoxy group.

4. Electrolyte according to claim 1, characterized in that Ri is an n-pentafluoropropoxy group.

5. Electrolyte according to one of the preceding claims, characterized in that R _{2 is} a trifluoromethyl group and R ₃ ,

R ₄ and R5 are H.

6. Electrolyte according to one of claims 1 to 4, characterized in that R ₂ , R ₃ , R4 and R5 are H.

7. Electrolyte according to one of the preceding claims, characterized in that the at least one phosphite is contained in a proportion of 0.1-15% by weight, based on the total weight of the organic solvents.

8. Use of an electrolyte in a lithium secondary battery at a voltage greater than 4.2 V, the electrolyte comprising: at least one lithium salt; at least a first and a second organic

Solvents, wherein the first organic solvent has a dielectric constant of more than 90 (at 40 ° C) and the second organic solvent has a boiling point of less than 110 ° C; and at least one phosphite of the formula (I)

wherein Ri is a fully or partially halogen-substituted, linear or branched C2-C2o ^_ alkyl group and R2, R3, R4 and R5 are independently selected from H or an unsubstituted or completely or partially

halogen-substituted, linear or branched Ci-C2o ^_

Alkyl group.

9. lithium secondary battery comprising a cathode; an anode; a separator; and the electrolyte according to one of claims 1 to 7.

10. Lithium secondary battery according to claim 9, characterized

characterized in that the proportion of phosphite in the electrolyte is 0.1-15% by weight, based on the total weight of the organic solvents.